This invention relates to misfire detection devices for internal combustion engines.
Various systems are already proposed for detecting misfires of internal combustion engines. For example, Japanese Laid-Open Patent (Kokai) No. 58-19532 proposes a device which includes a crank angle sensor and detects the rotational speed at an early and a later stage within the expansion stroke of the respective cylinders of an internal combustion engine. An occurrence of misfire is detected when the rotational speed difference between the early and later stages within the expansion stroke is less than a predetermined level.
The unburnt gas generated by the misfires of the internal combustion engines not only pollute the atmosphere but may react with the catalyst and, creating an abnormally high temperature, burn out the catalyst. Thus, it is necessary to detect an occurrence of misfire as early possible and to repair the malfunctioning parts.
The above conventional method of misfire detection, however, has the following disadvantage. Namely, although, for the purpose of enhancing the detection sensitivity, the rotational speed change over such an interval is detected where the variation of the rotational speed is greatest, the device detects the rotational speed change or deviation at fixed angles of the crank shaft. Thus, the sensitivity of the device is reduced when the ignition timing and hence the interval of time where the rotational speed change is greatest are shifted. It is noted that the ignition timing varies over an operational range of about 50 degrees width, from about 40 degrees before top dead center (BTDC) to 10 degrees after top dead center (ATDC) depending on the rpm and the load of the engine.